1. Field of the Invention
The present invention relates to an electroluminescent device.
2. Description of the Related Art
Electroluminescence (EL) is the emission of light from a phosphor layer due to the application of an electric field. Electroluminescent devices (EL) have utility as lamps and displays. FIG. 1 is a schematic cross-sectional view of a conventional electroluminescent device, which includes a bottom substrate 100, a first electrode layer 101 formed on the bottom substrate 100, a phosphor layer 102 formed on the first electrode layer 101, and a second electrode layer 103 formed on the phosphor layer 102. An AC power is connected to the first electrode layer 101 and the second electrode layer 103 to drive the electroluminescent device. In case of yellowish light emission, the phosphor layer 102 can be formed of sintered pellets of ZnS (parent material) doped with Mn (illuminating centers) as a dopant by way of electron beam vapor deposition, sputtering, screen printing, spin coating or ink-jet printing etc. As other sintered pellets for the phosphor layer 102, sintered pellets of ZnS doped with TbF3 or TbP as a dopant for green light emission, those doped with TmF3 for blue light emission are also available. In lamp applications, both of the first electrode layer 101 and second electrode layer 103 take the form of continuous layers, thereby subjecting the entire phosphor layer 102 between the two electrode layers to the electric field. In a typical display application, the first electrode layer 101 and second electrode layer 103 are suitably patterned with electrically address lines defining row and column electrodes (not shown). Pixels are defined where the row and column electrodes overlay. Upon applying an electric field onto the electroluminescent device, electrons from the first electrode layer 101 are accelerated by the electric field as they pass through the phosphor layer 102. The outer-shell electrons of the illuminating centers of the phosphor layer 102 are collided with the accelerated primary electrons, causing the outer-shell electrons transferring to the conduction band of the parent material to form free electrons, and the illuminating centers are ionized. The free electrons and ionized illuminating centers are recombined, and the energy difference there between are released in a form of light.
The conventional electroluminescent device still has problems such as a high driving voltage and low emission brightness. Thus, research and development of the electroluminescent device have been extensively made for improvement of light emission characteristics.